1. Field of the Invention
This invention generally relates to integrated circuit fabrication and, more particularly, to an imaging reticle employing a combination of grayscale and sub-resolutional features.
2. Description of the Related Art
It has proven difficult to form a smooth slope in photoresist over a long distance, at a low angle. Conventional lithography, using a binary pattern of fully opaque and fully transparent regions, can only crudely do the job. This form of lithography produces a sloped transmission change in discrete steps (see FIG. 8). Light transmission is also modified by the diffractive nature of the light and its unresolved capture through the imaging system (diffraction limited regime). Sub-resolutional patterns are used in this regime. The main problem of producing a smooth sloped photoresist profile is the lateral dimension. If the lateral dimension is too large, greater than say 100 microns, then the change possible due to resolutional patterning is less than the full lateral dimension. This is due to a discrete increment in the patterning.
Grayscale technology has inherent characteristics that lend themselves to a gradual change in transmission from dark to light, which can be used to address the problem of producing a smooth slope of photoresist. Canyon Materials, Inc. is a company that makes grayscale reticles by use of an E-Beam tool and a High Energy e-Beam Sensitive (HEBS) film that changes the transmission of light with a corresponding dose of electrons from the E-Beam tool. The problem with this approach is that the cost of a reticle is high.
FIG. 1 is a diagram depicting a multilevel grayscale mask and a resulting photoresist pattern (prior art). Conventionally, a slope is formed in photoresist by layering grayscale material. The film thickness (FT) is proportional to the dose of light incident on the film. In this case the film is a positive acting photoresist, (“whatever shows goes”). Points A, B, C, D, and E are cardinal points representing unpatterned layers of Ti, in this case there are 4, 3, 2, 1, or no layers of Ti. Thus, dose A forms a photoresist (PR) thickness as a result of being attenuated by grayscale layers L(1), L(2), L(3), and L(4). Dose D forms a lesser PR thickness since light only passes through layer L(1) of the reticle. Although the layered approach is cheaper than using an E-beam tool, the multiple process steps required to fabricate such a reticle are complicated. Further, unless an impractical number of thin layers are used, the slope formed by such a reticle in the PR is not gradual, but rather, consists of relatively large discrete steps.
It would be advantageous if there was a cost-effective way of fabricating a grayscale reticle capable of forming gradual slopes in PR, at a low angle.